[en] YBa₂M₃O_9-δ (M: Fe, Co) systems with composition similar to the YBa₂Cu₃O₇ superconductor were synthesized. Two types of Y_1.2Ba₃Fe₃O_z (orthorhombic, a = b = 4.06 angstrom, c = 4.04 angstrom) and YBa₂Fe₃O_9-δ (tetragonal, a = 3.94 angstrom, b = 3.92 angstrom, c = 3.90 angstrom) with perovskite-type structure were obtained for the iron oxide system, while YBa₂Co₃O_9-δ perovskite (cubic, a = 4.14 angstrom) was obtained for the cobalt oxide system. The conductivities and the Seebeck coefficients were measured in the temperature range from 650C to room temperature. The carrier densities and the mobilities were calculated by using a small polaron hopping model. All the samples were p-type semiconductors and showed hopping conduction. The mobility of holes was much larger for YBa₂Co₃O_9-δ than for YBa₂Fe₃O_9-δ, although the unit cell is larger for the former oxide than for the latter. This is explained through the larger overlapping integral of the σ* hopping band formed in YBa₂Co₃O_9-δ than the π* hopping band formed in YBa₂Fe₃O_9-δ

[en] Electronic and silver ionic conductivities in some silver-containing oxides with perovskite and related structures, such as AgNbO₃ (perovskite), AgSbO₃ (pyrochlore-related structure), and Ag-doped Bi-(Pb)-Sr-Ca-Cu-O-system superconductor, were measured by four-probe techniques using Pt electrodes and Ag-β double-prime-Al₂O₃ electrodes with silver ionic conduction, respectively. Both AgNbO₃ and AgSbO₃ were n-type semiconductors and their electronic conductivities increased with an increase in the silver content. The electronic conductivity of AgSbO₃ was higher than that of AgNbO₃, due to the presence of a large amount of oxygen vacancies. Sb⁴⁺ and/or Sb³⁺ in AgSbO₃, and Nb⁴⁺ in AgNbO₃ will act as a donor. Silver ionic conductivities were always lower than the electronic conductivities brought about by the above donors for both AgNbO₃ and AgSbO₃ ceramics. The activation energies of silver ionic conductivity were in the ranges of 1.01-1.58, 0.78-0.90, and 0.94-1.12 eV for AgNbO₃, AgSbO₃, and Ag-doped Bi-system superconductors, respectively. The silver ion was assumed to move via the Ag sites in the bulk for AgNbO₃, but to move in the grain boundaries for AgSbO₃ and the Bi-system superconductor. The mechanisms of the electronic and silver ionic conductions are discussed for the present oxide ceramics

[en] Low temperature sintering of YBa₂Cu₃O_y ceramics by various Ag compounds was studied by in situ resistivity measurement using a four-probe method and silver ionic conductivity measurement using four silver ion conduction electrodes of Ag-β double-prime-Al₂O₃. Even silver ionic compounds promoted sintering, indicating that the Ag⁺ ion promotes sintering in the grain boundaries. The activation energies of Ag⁺ ionic conductivities of the Ag-added samples (0.2 - 1.4 eV) were much lower than those in the YBa₂Cu₃O bulk (2.3 - 2.5 eV), indicating that the Ag⁺ ion easily diffuses in the grain boundaries at low temperature

[en] Perovskite phases in the system, La sub(1-x)Ca sub(x)FeO sub(3-α) were prepared with La²O³, CaCO³, and Fe²O³ by firing in air and in vacuo. The compositions of samples fired in vacuo and in air are represented as La sub(1-x)Ca sub(x)FeO sub(3-x 2) and La sub(1-x)Ca sub(x)Fe sub(1-y)sup(3+)Fe sub(y)sup(4+)O sub(3-x/2+y/2), respectively. That is, samples fired in vacuo contain some oxygen vacancies and no tetravalent iron; in contrast, samples fired in air contain both oxygen vacancies and tetravalent iron in the structures. The electrical conductivities of these synthesized oxides depended extensively upon the content of tetravalent iron. For instance, the conductivity of the sample x = 0.6 fired in air was larger by 10₆ than that of the sample fired in vacuo. In this system, except for the two terminal compositions of x = 0 and x = 1.0, the values of activation energy for conduction are considerably small, and from the results of thermo-electromotive force measurement, the charge carrier was positive. Furthermore, the conductivity increased somewhat with time during the conductivity measurement by the direct-current method. These facts suggest that the electrical conducted would not be ionic but electronic. The electrical conduction would then be carried out by the so-called hopping mechanism by which the positive charge is transferred. (author)